Scroll compressors comprise a pair of scrolls, a fixed scroll and an orbiting scroll. The two scrolls are nested or interengaged within the compressor in an opposed orientation, that is, with the spiral scroll wall flanks facing in opposite directions. The flanks of the scroll walls engage at rolling contact lines which form gas pockets that continually expand and contract in toward the center to compress refrigerant vapor. The gas pockets are sealed both by the close engagement of the rolling contact lines and by the axial engagement of the terminal edges of each scroll wall with an opposed flat surface of the other scroll. The axial end sealing of the pockets is assisted by tip seals inset into grooves in the terminal edges of the scroll walls.
A typical orbiting scroll is shown in FIGS. 1 through 4, and indicated generally at 10. The non-illustrated fixed scroll is essentially identically configured, at least in those aspects relevant to the subject invention. A base plate 12 has a flat sealing surface 14 from which a scroll or involute wall 16 extends to a terminal edge 18. As can be clearly seen, the edge 18 is much thicker at the inboard end (T1) than at the outboard end (T2), due to the fact that the gas pocket pressure, which acts in the direction of the arrows, is much greater at that point than further outboard. Each scroll's terminal edge 18 carries a spiral shaped tip seal 20 that is inserted nearly flush into the surface of the edge 18. Very little of the original surface of edge 18 remains to either side of seal 20, so it is not feasible to machine the corners of edge 18 with anything but a very small, sharp radius, with one exception noted below.
Referring specifically to FIG. 2, seal 20, in modern designs, is typically an injection molded plastic material with superior lubricity, such as the commercially available high temperature plastic PPS. It has a width slightly less than the width of its mounting groove, and a thickness slightly less than the depth thereof. The conventional tip seal 20 is square cornered in cross section, and does not run all the way to either end of the edge 18, since the mounting groove therefore cannot run all the way to the end. For the same reason, the tip seal 20 is obviously narrower than the edge 18 throughout, since the mounting groove must be narrower.
Referring next to FIG. 4, further details of the scroll 10 are illustrated. The surfaces of the scroll wall 16 and the sealing surface 14 must obviously be carefully machined to shape in order to assure tight sealing of the gas pockets, and adequate compression. The machining and shape of the transition between the flank surface of the scroll wall 16 and the base plate flat surface 14 is also critical to proper operation. That transition, rather than being truly sharp cornered, has a slight concave curvature that is generally referred to as a fillet radius. Basic mechanics teaches that a too sharp fillet radius can create a stress riser and potential cracking. Since the higher pressure in the gas pocket toward the inboard end of the scroll wall 16 puts the fillet radius there in greater tension, a larger fillet radius R2 of approximately 1 mm is used over the length thereof so marked. The rest of the fillet radius around both sides of the scroll wall 16 is sharper, with a concave radius of approximately 0.2 mm. The machining sequence is as follows. First, an end mill with the larger, R2 radius is run around the entire length of the transition of the flank or scroll wall 16 to the surface 14 on both sides thereof. The larger radius R2 would interfere with the sharp cornered terminal edge 18 of the opposed scroll, however. In order to allow proper matching of terminal edge to transition, a second end mill with the sharper, R1 radius is next run around almost the entire length of the transition again, cutting material out of it and leaving the sharper radius R1 behind. The second tool is pulled radially out and away from the inboard side of the transition before reaching the inboard end, however. As seen in FIG. 4, this leaves a distinctive "stepped" section at B, with the remainder of the corner up to the thicker inboard end retaining the original, larger radius R2. Finally above that length of the transition corner that has either the larger radius R2 or the "step", a corresponding length of the otherwise sharp cornered edge 18 is chamfered off at C, with yet a third tool. The length C is the only non sharp corner on the edge 18, and it can be successfully cut, since the edge 18 is wider at that point.
Referring next to FIG. 8, the operation of a conventional pair of scrolls is illustrated Corresponding surfaces of the other, fixed scroll are given the same number primed. Gas pocket pressure is able to leak slightly under the seal 20, which is biased out slightly proud of the surface of the edge 18 to tightly engage the opposed flat sealing surface 14. The degree of clearance around the seal 20 is exaggerated for purposes of illustration. Sealing is assisted, and direct metal to metal contact with the flat surfaces 14-14' is avoided. This is only true where the seals 20-20' are disposed, however, and they do not run end to end of the edges 18-18'. The convex corners of the edges 18-18' do directly face the concave transition corners of scroll wall 16 (16') to surface 14 (14') at all points, metal to metal. This is so, because the seals 20-20' do run the full width of the edges 18-18', and cannot. since they are inset into a groove. Those convex corners must be given a radius that is equal to or greater than the concave corner so as to avoid the possibility of metal to metal interference contact. And since the convex corner of edge 18 is machined out of very narrow border of metal remaining after the mounting groove is cut, it must have a small, sharp radius, as must the matching concave transition corner. While this common configuration has worked well in practice, the need for three different tools and three different operations to produce the two radii, R1 and R2, plus the chamfer C, adds time and expense. There is no obvious way to eliminate the three total steps, however, since the inboard end of the scroll wall needs the larger radius, but that larger radius cannot be applied all the way around.